ISLAMABAD (Reuters) - Opposition leader Imran Khan opened negotiations Wednesday with the Pakistani government, a lawmaker from his party said, in an effort to end protests against the prime minister and overcome a political impasse.

The announcement came the day after Pakistan's powerful military said the two sides should engage in dialogue and warned that key government institutions were under its protection.

Former cricket star Khan and cleric Tahir ul-Qadri, who controls a network of Islamic schools and hospitals, have been leading protests in the capital Islamabad since Friday.

Both want Prime Minister Nawaz Sharif to resign over allegations of corruption and election rigging. Sharif, who won the last election by a landslide, has refused.

The civilian government is seeking to cement its authority over the nuclear-armed nation of 180 million, after decades when the nation swung between democracy and military rule.

Supporters of Mohammad Tahir ul-Qadri, Sufi cleric and leader of political party Pakistan Awami Tehr …

The South Asian nation is also plagued by high unemployment, daily power cuts and a Taliban insurgency, while anti-Western and violent sectarian groups are gaining strength.

Most protesters on the streets of Islamabad say they are demonstrating against government corruption, which they blame for the country's widespread poverty.

In a speech to thousands of supporters, Khan said he would not stop protesting until Sharif quit, but backed away from a previous pledge to storm the prime minister's house.

"I thought I would take you all to the prime minister's house today," he said. "(But) I thought his heart is already in bad shape. If I ask my followers to go in that direction (of his house) and he has heart attack - I cannot do that."

Lawmaker Asad Umar from Khan's party said that they had six demands. One was Sharif's departure, and the five others concerned governance and election reforms, he said.

On Tuesday night, protesters used cranes and bolt cutters to dismantle police barricades and surround parliament. On Wednesday, cleric Qadri urged the crowd to barricade lawmakers and the prime minister inside as they met to discuss the crisis.

"Don't let all those inside come out and don't let anyone go in," he told supporters.

"AFFRONT"

His exhausted followers, some carrying blankets or colourful umbrellas, were resting in the shade on the grass on Constitution Avenue when he spoke. But they immediately rose to block the entrance to parliament.

Supporters of Mohammad Tahir ul-Qadri, Sufi cleric and leader of political party Pakistan Awami Tehr …

The military, which often acts as an arbiter when it is not ruling directly, broke its silence to call for a political solution to the crisis.

"(The) Situation requires patience, wisdom and sagacity from all stakeholders to resolve (the) prevailing impasse through meaningful dialogue in (the) larger national and public interest," military spokesman General Asim Bajwa tweeted as the protesters approached parliament.

Last month, the civilian government made the military officially responsible for the security of top government offices. All the protesters have been careful not to offend the military, which is Pakistan's most powerful institution.

One of the country's other power centers is the activist judiciary, which waded into the fray on Wednesday when Chief Justice Nasir ul-Mulk summoned Khan and Qadri to appear on Thursday over a petition filed against their protests.

Pakistan's top courts can declare an interest in any case or accept a complaint from any petitioner. They can also charge those who question their decisions with contempt of court.

Khan wants Sharif to step down because he believes the prime minister rigged last year's polls. Sharif won the election by a landslide, taking 190 out of 342 seats in the national assembly.

The ballot was the first democratic transfer of power in Pakistan's history and also propelled Khan from a fringe player to head of the third-largest legislative bloc in the country.

Qadri wants Sharif to step down because he says the system is corrupt. He has promised free housing for the homeless, and welfare and subsidized food and electricity for the poor.